docs: fix stale-live docs and drop obsolete Obsidian spec

Refreshes the docs that were actively misleading about the current
FastAPI + React + service-registry app, and deletes one obsolete design.

- CONTRIBUTING.md: full rewrite — Streamlit-era guidance replaced with
  the current backend (ruff/pytest, src/ layout) + frontend (npm
  lint/build/test) workflow, service-registry model, and shadcn/Tailwind
  stack. Mirrors AGENTS.md.
- README.md: removed the non-existent /addons/:addonId route (Services
  page is current); fixed the per-machine Jellyfin wording; replaced the
  py_compile dev snippet with ruff + pytest / npm lint+build+test.
- backend/README.md: updated the structure tree (removed deleted
  clients/resources.py; added routers backups/services/tasks/widgets,
  integrations/, models/, widgets/, workers/); dropped the "starts the
  collector" sentence (MonitoringPoller is decommissioned).
- frontend/README.md: corrected the uvicorn module path
  (main:app -> media_library_viewer_api.main:app).
- Deleted docs/superpowers/specs/2026-05-08-obsidian-documentation-design.md
  (Obsidian vault never built; stack refs MUI/D3/AG Grid all removed).

Historical docs (MIGRATION_PLAN, superpowers backup-monitoring, the
bannered design/runbook/context files) deferred to a later banner pass.

docs/superpowers/specs/2026-05-08-obsidian-documentation-design.md

@@ -1,53 +0,0 @@
-# Obsidian Documentation Structure for Manage (Media Library Viewer)
-
-**Date:** 2026-05-08
-**Status:** Approved
-
-## Purpose
-
-Create a comprehensive, interconnected Obsidian documentation vault for the Manage project (media library viewer application). The documentation targets all audiences: future developers, contributors, operators, and deployers.
-
-## Structure
-
-### Map of Content (MOC)
-- `manage/Overview.md` — Central hub with wikilinks to all documentation areas
-
-### Architecture & Overview
-- `manage/Architecture.md` — System design, data flow, tech stack
-- `manage/Directory Structure.md` — Annotated codebase layout
-
-### Backend
-- `manage/Backend/API Endpoints.md` — Complete REST endpoint map
-- `manage/Backend/Configuration.md` — Settings, env vars, OIDC
-- `manage/Backend/Services.md` — Core services: settings store, media index, poller, mail
-- `manage/Backend/Clients.md` — External integrations: Jellyfin, Jellyseerr, SSH
-
-### Frontend
-- `manage/Frontend/Pages & Routing.md` — Routes, page components
-- `manage/Frontend/Components.md` — Reusable components
-- `manage/Frontend/State & Data.md` — Hooks, QueryClient, data fetching
-- `manage/Frontend/Auth & Theme.md` — OIDC auth, MUI theme
-
-### Deployment
-- `manage/Deployment/Production.md` — Docker Compose, Traefik, TLS
-- `manage/Deployment/Development.md` — Dev workflow, hot reload
-
-### Operations
-- `manage/Operations/Machine Management.md` — SSH keys, collectors
-- `manage/Operations/Monitoring.md` — Poller, metrics
-- `manage/Operations/Tasks & Jobs.md` — Saved tasks, job templates
-
-### Development
-- `manage/Development/Setup.md` — Getting started for backend + frontend
-- `manage/Development/Testing.md` — Test structure and commands
-- `manage/Development/Contributing.md` — Conventions, PR workflow
-
-## Cross-Linking Conventions
-- Every note uses YAML frontmatter with `tags` and `aliases`
-- Related docs linked via `[[wikilinks]]`
-- Callouts for warnings, tips, and notes
-
-## Technologies Referenced
-- Backend: Python 3.11+, FastAPI, SQLite, Paramiko, PyJWT
-- Frontend: React 19, TypeScript 6, Vite 8, MUI 9, D3 7, AG Grid
-- Infra: Docker, Traefik, Authentik/OIDC